1. Technical Field
Aspects of the present invention relate to a paper feed unit that separates a sheet of paper from a plurality of sheets of paper stacked in a paper tray of a facsimile apparatus, a printer, a copying apparatus or the like and feeds the separated sheet of paper one by one. Aspects of the present invention also relate to a paper separating unit included in the paper feed unit, and to an image forming apparatus in which the paper feed unit is incorporated.
2. Description of Related Art
The paper feed units being currently used include a type that separates a sheet of paper from a plurality of sheets of paper stacked in a paper tray, using friction provided by a paper feed roller and a separating pad (or separating piece) that presses the back surface of the paper against an outer circumferential surface of the paper feed roller, thus to feed the separated paper one by one.
More specifically, the paper feed roller is disposed such that an outer circumferential surface thereof contacts at least the topmost paper of the plurality of sheets of paper stacked in the paper tray, so that, when the paper feed roller rotates, the topmost paper is sent forward. The separating pad is supported by a holder, and the holder is biased toward the paper feed roller by a coil spring or the like, so that the separating pad is pressed against the outer circumferential surface of the paper feed roller. Under such structure, though a plurality of sheets of paper is forwarded by the rotation of the paper feed roller toward the contact interface between the separating pad and the paper feed roller, only one sheet of paper is separated and forwarded according to mutual relationship (in magnitude) of frictional forces acting between the outer circumferential portion of the paper feed roller and the paper, between the sheets of paper, and between the paper and the separating pad.
The paper feed unit thus designed is incorporated in various image forming apparatuses such as facsimile apparatuses, printers, copying apparatuses and so on, to feed the sheets of paper one by one to an image forming unit.
With the foregoing paper feed unit however, the paper feed roller causes a plurality of sheets of paper to move forward out of the paper tray, whereas normally each of the sheets of paper is separated by the interaction of the sheets of paper with the separating pad. However, when a large number (for example more than 10 sheets) of sheets of paper are forwarded and rush to the separating pad at a time, the separating pad may be parted from the paper feed roller by the thickness of the sheets of paper, thereby resulting in the feeding of a plurality of unseparated sheets of paper at a time. This is referred to as a “double feed”.
To eliminate such a problem, a structure of the holder and the separating pad that allows the separating pad to elastically deform (recede toward the holder) when many sheets of paper rush to the separating pad has been proposed. The structure operates so as to deposit the sheets of paper in a space created by the elastic deformation and thus to prevent the double feed, as disclosed in Japanese Patent Provisional Publication No. 2005-255385 (hereafter, referred to as JP 2005-255385).
FIG. 7 illustrates the paper feed unit according to JP 2005-255385. The paper feed unit 100 is designed such that a plurality of sheets of paper 104 stacked in a paper tray 102 is individually forwarded by rotation (counterclockwise in FIG. 7) of a paper feed roller 106. The sheets of paper 104 in the paper tray 102 are butted to a contact surface of a paper stage 108 downwardly inclined toward the paper feed roller 106, and hence the sheets of paper 104 tend to intrude between the paper stage 108 and the paper feed roller 106 by the papers' own weight. Here, the front edge (closer to the paper feed roller 106) of the paper stage 108 is farther ahead of the outer circumferential surface of the paper feed roller 106 by a gap a, when viewed from an upstream side in the paper feed direction as shown in FIG. 10. Accordingly, the sheets of paper 104 stacked in the paper tray 102 stop once between the paper stage 108 and the paper feed roller 106. The paper feed roller 106 is caused to rotate when a driving force of a motor (not shown) is transmitted to the rotating shaft of the paper feed roller.
Below the paper feed roller 106, a paper separating unit including a holder 110 that supports a separating pad 112 which is mounted thereon. The holder 110 can swing about a swinging fulcrum P, and is biased by a coil spring 116 so as to press the separating pad 112 against the outer circumferential surface of the paper feed roller 106. The holder 110 includes a recessed portion 114 located on an upstream side in the feed direction, and the separating pad 112 has a predetermined region thereof on a downstream side in the feed direction supported by a supporting base 110a, and an end portion on an upstream side in the feed direction supported by an upper end supporter 110b. 
With the paper feed unit 100 thus configured, normally the paper 104 is forwarded as shown in FIGS. 8A to 8D. Referring first to FIG. 8A, when the sheets of paper 104 are stacked in the paper tray 102, the sheets of paper 104 are orderly placed between the paper stage 108 and the paper feed roller 106 because of the sheets of paper 104s' own weight. Once the paper feed roller 106 starts to rotate under this state, a plurality of sheets of paper 104 rush to the separating pad 112 as shown in FIG. 8B. Accordingly, the plurality of sheets of paper 104 is induced to intrude between the separating pad 112 and the paper feed roller 106, however as shown in FIG. 8C, the separating pad 112 recedes toward the recessed portion 114, so that the plurality of sheets of paper 104 is detained in a space created by the receding motion, though the holder 110 itself does not swing. The sheets of paper 104 thus detained by the separating pad 112 are separated into each individual sheet by the rotation of the paper feed roller 106, thus to be forwarded one by one.
However, in the case where a larger (unexpected) number of sheets of paper 104 are forwarded toward the separating pad 112, such as when a frictional force between the sheets of paper 104 is excessively large or when the sheets of paper 104 are improperly set by a user so that an excessive amount of sheets of paper 104 are in contact with the paper feed roller 106, some of the many sheets of paper 104 may directly contact the holder 110, thus causing the holder 110 to swing, by which the separating pad 112 is separated from the paper feed roller 106.
FIGS. 9A to 9D illustrate the details of such phenomenon. When the paper feed roller 106 rotates to start feeding the sheets of paper 104 (FIG. 9A), if an unexpected number of sheets of paper 104 are fed at a time, a part of those sheets of paper directly contacts the holder 110 as shown in FIG. 9B. If the paper feed roller 106 continues to rotate so as to feed the sheets of paper 104 into a narrower location (the contact interface between the separating pad 112 and the paper feed roller 106), the load being applied to the sheets of paper 104 increases and hence the frictional force between the sheets of paper also increases. This applies an unexpectedly heavy load to the holder 110, by which the holder 110 is caused to swing, thus separating the separating pad 112 from the paper feed roller 106 as shown in FIG. 9c. 
When a gap is thus created between the separating pad 112 and the paper feed roller 106, the conventional problem is reproduced, as the sheets of paper are double-fed without being separated into each sheet. This problem is also incurred when the sheets of paper 104 are butted to a region that is difficult or impossible to be elastically deformed by the weight of the sheets of paper, such as the region on the separating pad 112 supported by the upper end supporter 110b, in addition to a case where the sheets of paper 104 are directly butted to the holder 110. In other words, when the sheets of paper 104 are butted to the separating pad 112 at a position that does not cause the separating pad 112 to recede (elastically deformed toward the recessed portion 114), the large load of the sheets of paper 104 is directly applied to the holder 110, and resultantly the holder 110 is caused to swing.
As already seen in FIG. 10, since the front edge (closer to the paper feed roller 106) of the paper stage 108 is farther ahead of the outer circumferential surface of the paper feed roller 106 by the gap α, a central portion of the sheets of paper 104 slightly swells toward the paper tray 102 when the sheets of paper are fed by the paper feed roller 106. This is another factor that induces the sheets of paper 104 to be directly butted to the holder 110.